Heretofore, the treatment of exhaust gases to achieve the above art has been done either by an absorption column filled with packing material as shown in FIG. 1 or by an absorption tower of venturi type in which liquid drops are finely divided at a high pressure loss for absorption purpose.
Referring to FIG. 1, which is a flow sheet of exhaust gas treatment in accordance with a conventional process, there is shown a conventional absorption column including an exhaust gas inlet 1, packing sections 2 filled with packing material such as Raschig rings, and a tank 3 at the bottom. Numeral 4 designates a circulating pump for a chemical liquid to be supplied at 5, 6, and 7 for circulation through the column, the supply port 5 being formed in a line on the discharge side of the circulating pump, the port 6 on the suction side, and the port 7 in the tank of the absorption column. An outlet 8 for draining the circulating liquid is usually provided in the line on the discharge side of the pump 4. The column has a cleaned exhaust gas outlet 9 at the top and an industrial water inlet 10 at the bottom.
An exhaust gas stream enters the absorption column at the inlet 1, passes through the packing sections 2, and leaves the vessel at the outlet 9. A chemical liquid is supplied to the column through the supply port at 5, 6, or 7. Industrial water is supplied through the inlet 10. The circulating liquid collected at the bottom cone or tank of the absorption column is taken out by the circulating pump 4 and is reintroduced into the column, at a point above the packing sections 2, and is sprayed into contact with the exhaust gas through the packing sections 2 to absorb harmful gaseous substances from the exhaust gas stream.
Ordinarily, for example, according to a hydrochloric-acid recovery apparatus, the waste liquid from the means descaling the surface of rolled steel plate in this apparatus contains ferrous and ferric chlorides, and the iron oxides are removed by decomposing the liquid at elevated temperature by a fluidized-bed reactor or by other means. Hydrogen chloride, HCl, that is simultaneously produced is recovered as aqueous HCl by water washing in the apparatus. For enhanced economic advantages, a compact recovery equipment is usually used to recover the hydrochloric acid in a high concentration, and therefore the HCl gas concentration at the outlet of the recovery apparatus ranges from hundreds to one thousand and several hundred parts per million. The emissions from the apparatus thus contain large proportions of harmful gases, such as HCl and Cl.sub.2, soot and dust. They cannot be released as such to the atmosphere, but should be disposed of somehow or other for environmental reason.
In view of this, we made diversified attempts to develop an apparatus capable of treating exhaust gases rich in such noxious gases as HCl and Cl.sub.2 that result from the process of recovering iron oxides and hydrochloric acid from the waste acids leaving acid pickling lines. As a result, it was found useful to scrub the emissions with an absorbent liquid which uses as the absorber an alkali metal or alkali earth metal, e.g., lime slurry, caustic soda solution, or ammonia solution. However, new problems arise here. In the process for recovery of hydrochloric acid from exhaust gases, water scrubbing is used to recover the objective substance in the form of aqueous hydrochloric acid, and naturally the gases during the recovery process are saturated with water and at a temperature of 75.degree. C. or upwards, the temperature remaining unchanged at over 75.degree. C. inside the gas scrubber. The HCl gas concentration in the treated gas discharge to the atmosphere is to be limited within several ppm by law and regulations. The quantity of water to be drained should also be minimized in the light of the restrictions on COD chemical oxygen demand, SS, suspended solid etc. in the effluent provided for in the law; draining much water by abandoning the chemical liquid in a one-pass operation is, of course, infeasible. If, for example, the liquid-gas ratio in the packing zone is in the usual range of 2.0-4.0 l/m.sup.3 N, the quantity of water to be drained by the one-pass system will be as much as 2.0-4.0 l per m.sup.3 N of the exhaust gas per hour.
For these reasons, it has been common practice to recycle and reuse the chemical liquid.
Generally, the exhaust gases from coal-fired boilers and refuse incinerators, with moisture contents of 8-30% and at temperatures of about 150.degree.-180.degree. C., are changed in scrubbers to be gases with saturation humidity at temperatures at or below 75.degree. C. In order to dispose of the SO.sub.2, HCl, HF, and Cl.sub.2 in these exhaust gases, the scrubbers are designed to have absorption capacity coefficients (absorption capabilities) in the range of 800-900 kg/mol/m.sup.3.hr.atm, and hence are compact in construction. However, the exhaust gases at extremely high temperatures or with high humidity values will still maintain high temperatures within the scrubbers. When harmful gaseous substances in low concentrations are to be absorbed from an exhaust gas charge at a high temperature in a scrubber, the resistance on the part of the liquid will be so high that the absorption capacity coefficient (absorbability) of the scrubber will decrease and, to make up for this, the apparatus will have to be large in size. Experiments indicated that, where the temperature inside the scrubber was 84.degree. C. and the hydrochloric acid gas concentration at the outlet was 5 ppm, the absorption capacity coefficient (absorbability) was at most 100-200 kg-mol/m.sup.3.hr.atm, and the scrubber had to be from five to six times as large as the normal size. In another experiment a heat exchanger was employed to lower the exhaust gas temperature in a scrubber and an absorption test was conducted with a circulating liquid at below 75.degree. C. This installation proved of no practical use because of the large size of the heat exchanger and ancillary arrangements and also of the oversize of the utilities required.